As a filter for trapping and removing particulate matter contained in dust-containing fluid such as exhaust gas discharged from a diesel engine (diesel particulate filter (DPF)), or a catalyst carrier for supporting a catalyst component for purifying a toxic substance contained in exhaust gas, a porous honeycomb structure including cell partition walls (ribs), which form a group of adjacent cells, and a honeycomb outer wall, which encloses and holds the outermost circumferential cells located at the outer circumference of the cell composite, has been widely used. As a material for the honeycomb structure, refractory silicon carbide (SiC) has been used.
A DPF using a regeneration method in which an oxidizing catalyst is loaded on a DPF and deposited particulates are oxidized and burned to effect continuous regeneration (catalytic regeneration DPF) has also been developed.
As such a honeycomb structure, a porous silicon carbide catalyst carrier having a honeycomb structure obtained by forming silicon carbide having a predetermined specific surface area and containing impurities as a starting material into a desired shape, drying the formed product, and firing the dried product in the temperature range of 1600 to 2200° C. has been disclosed (see JP-A-6-182228, for example).
In the sintering mode (necking) of the catalyst carrier disclosed in JP-A-6-182228 utilizing the recrystallization reaction of silicon carbide particles, the silicon carbide component is evaporated from the surface of the silicon carbide particles and condenses at the contact section (neck section) between the particles, whereby the neck section is grown to obtain a bonding state. However, since an extremely high firing temperature is required to evaporate the silicon carbide, cost is increased. Moreover, since it is necessary to fire the material having a high coefficient of thermal expansion at a high temperature, firing yield is decreased.
In the case of manufacturing a filter having a high porosity (particularly a filter having a porosity of 50% or more) by sintering utilizing the recrystallization reaction of the silicon carbide particles, the growth of the neck section is hindered since the sintering mechanism does not sufficiently function, whereby the strength of the filter is decreased.
As a related-art technology for solving these problems, a porous honeycomb structure including refractory particles (particularly silicon carbide) as an aggregate and metal silicon and a method of manufacturing the same have been disclosed (see JP-A-2002-201082, for example). According to such a manufacturing method, a porous honeycomb structure can be inexpensively manufactured at a relatively low firing temperature, and the resulting porous honeycomb structure has characteristics such as a high porosity, high thermal conductivity, and high strength.
In order to improve the strength and the oxidation resistance (during abnormal combustion or the like) of the honeycomb structure, a technology of forming an oxygen-containing phase on the surfaces of silicon carbide and metal silicon by using a method of (1) oxidizing a silicon carbide raw material and metal silicon in air in advance or oxidizing the materials in a calcinating stage, (2) subjecting the product obtained after firing to a heat treatment in an oxygen-containing atmosphere, (3) coating the surface of the honeycomb structure using a solution containing silicon and oxygen, or the like has been proposed (see JP-A-2002-154882, for example).
In order to further improve the strength of the honeycomb structure in comparison with the above-mentioned related-art technologies, there has been proposed a method of adding an alkaline earth metal (calcium or strontium) having an eutectic point of 1200 to 1600° C. to an oxide film (silicon dioxide) on the surfaces of silicon carbide as an aggregate and metal silicon, and melting and removing the oxide film to improve wettability between the silicon carbide and the metal silicon, thereby thickening the joint section between the silicon carbide and the metal silicon (Japanese Patent Application No. 2002-61989). According to this method, a honeycomb structure having excellent strength can be obtained. However, the addition of calcium causes formation of a calcium silicate (alkaline earth metal silicate) crystal phase in the oxide phase after firing and dissolved in acetic acid used when causing the honeycomb structure to carry a catalyst, whereby a solution (solution in which catalyst is dissolved in acid) is contaminated. This problem was solved by replacing calcium with strontium. However, a strontium silicate (alkaline earth metal silicate) crystal phase is formed in the same manner as in the case of using calcium, depending the amount of strontium added and the firing conditions, and dissolved in acetic acid, resulting in poor acid (acetic acid) resistance.